# Finding output resistance

I am confused on how to find the output resistance of transistor amplifiers in general, but more specifically when it's a combination of different resistances.

Here's an example (concrete) of the source follower (from Sedra Smith 6th ed.): Could someone give me a purely analytical solution on how they came to the conclusion the output resistance $R_o$ is $\frac{1}{g_m} \parallel r_o$ in the small-signal model?

Beside, what is the general method to get the output resistance? Do we cancel $v_{sig}$? Do we replace the output terminal with a voltage source $V_o$ and calculate the thevenin resistance $\frac{V_o}{i_{o}}$ ? I'm not sure exactly what sources we keep and what we cancel, etc.

Thanks

• I noticed that on this unrelated question: electronics.stackexchange.com/questions/93428/… I gave what appears to be a reasonable answer yet you have neither accepted the answer nor raised a comment to ask further questions about my answer. What should I conclude? Jan 23, 2016 at 0:38
• That was long ago. At the time I wrote the question I wasn't even familiar enough with SE to know we had to chose answers. I just did. However at the time I was looking for a way to derive the QL/QC expresions from the resonance freq (i think?), I found it later myself so I didn't bother with the question. Just accepted it though, even if it was a partial answer. Jan 23, 2016 at 10:50

Okay. Griping about your text book over. Time to help you solve your problem. 